1. Field of the Invention
The present invention relates to an imaging device including a photoelectric conversion element. More particularly, this invention relates to the technology of improving the quality of a captured image when capturing a subject having high brightness.
2. Description of the Related Art
An imaging device such as a CMOS image sensor reads noise data (noise voltage) at respective photoelectric conversion elements during a reset period. For example, a correlated double sampling circuit (hereinafter also referred to as a CDS circuit) of the imaging device subtracts the noise data from pixel data which includes the read noise data, to thereby generate true pixel data. However, when a subject having high brightness is captured, the level of the noise data comes closer to the level of the pixel data due to the leakage of electric charges inside the imaging device. Since the level difference between the noise data and the pixel data becomes smaller, the brightness shown by the pixel data relatively lowers. In the case of capturing the sun, for example, the brightness of the sun becomes extremely low in the captured image, and the image of the sun becomes black. This phenomenon is generally referred to as black sun.
In Japanese Unexamined Patent Application Publication No. 2004-112740, the technique that a noise data level to be supplied to a CDS circuit is switched to a fixed voltage level when the level of the noise data becomes equal to or lower than a reference voltage level in order to prevent the black sun is disclosed. The fixed voltage level shows a standard black level when the brightness of the subject is zero, for example.
In the CMOS image sensor, the photoelectric conversion element is constituted of a photodiode and transistors. Threshold voltages of the transistors vary according to fabricating conditions of the imaging device (semiconductor). The above-described level of the noise data changes following the variation of the threshold voltages of the transistors. Meanwhile, in general, the above-described reference voltage is not influenced by the variation of the threshold voltages. Accordingly, the level of the noise data changes relatively to the above-described level of the reference voltage due to the variation of the threshold voltages. As a result of this, the problem arises that the noise data level and the fixed voltage level are not switched normally.
For example, when the noise data level becomes too high as compared to the reference voltage level, the noise data level cannot be switched to the fixed voltage level even though the phenomenon of the black sun is occurring. On the contrary, when the noise data level becomes low as compared to the reference voltage level, the noise data level is switched to the fixed voltage level at all times because it is determined that the phenomenon of the black sun is occurring at all times.
This kind of problem can be solved by generating a plurality of reference voltages by dividing resistance and the like, and switching between the reference voltages according to the threshold voltage of the manufactured transistor, for example. When the reference voltages are switched using this method, however, it is necessary to form a trimming circuit such as a fuse circuit inside the imaging device. Additionally, fuse cut process needs to be provided in test process for testing the imaging device. As a result of this, the cost of the imaging device increases.